This invention pertains to a method of forming relatively thick sheet metal products from superplastic sheet metal stock. More specifically, this invention pertains to a practical and rapid stretch forming method of forming such products using a plurality of relatively thin sheets of suitably deformable starting material.
Most automobile body panels are made by shaping low carbon steel or aluminum alloy sheet stock into body and panel shapes. The number of sheet metal pieces that must be formed and welded or otherwise attached together to form the vehicle body depends upon the design shape of the parts and the formability of the sheet metal. It is desirable, both from the viewpoint of manufacturing cost and fit and integrity of the assembled structural panels, to make the body from as few parts as possible. Other manufacturing operations are likewise affected by the complexity of a product shape that can be formed from the starting sheet metal. Thus, there is always an incentive to devise more formable metal alloys and better forming processes so that relatively few parts of more complex shape can be made and joined to make a car body or other product rather than joining a myriad of smaller, simpler pieces.
Superplastic titanium and aluminum sheet metal alloys have been identified and their use explored in the manufacturing of sheet metal products, especially those of complex configuration. Superplastic titanium alloys have been described for use in aerospace structures. The relatively high cost of titanium alloys and their slow superplastic forming cycles may be acceptable in aeronautical applications in order to save weight. Other industries usually look to less expensive alloys, such as aluminum alloys, for major component applications. Such industries also seek high productivity sheet metal forming processes.
Some investigations of superplastic aluminum alloys for automotive and boat applications have been undertaken. A common practice is to secure a sheet of the material at its periphery, heat it to a superplastic forming temperature, and apply gas pressure to one side to stretch the sheet against a forming tool. This is known as stretch forming. But difficulties have been encountered both in dealing with slow forming cycles and a typical substantial decrease in formability of sheets much thicker than about two millimeters (mm). It is observed the formability of superplastic sheet metal alloy stock often decreases significantly as thickness of the sheet increases over about 2 mm or so. The forming times are quite long and the sheet tears in regions of high elongation. The strength requirements of automotive body sections and the like may require thicker sections of complex configuration. The art has not found a practical way for superplastic aluminum alloys to be used in such applications.
This invention is based on the discovery that two or more relatively thin sheets of certain superplastic aluminum alloys can be stacked and stretch formed together. If desired, the two sheets can be formed without diffusion bonding between them. It was found that two identical layered thin sheets of a suitable magnesium-containing aluminum alloy could readily be formed with greater deformation or elongation than a single sheet of comparable thickness. In other words, it is possible and practical to form, by superplastic forming, preferably stretch forming, a more complex product shape (in the sense of degree of elongation or deformation) using two or more thin superplastic metal layers to obtain the strength of a unitary thick piece.
This practice is applicable, for example, to the family of known alloys of aluminum, titanium and other metals in which superplastic properties are attributed an extremely fine grain (usually less than 10 to 15 xcexcm) microstructure which appears at relatively low magnification as a pseudo-single phase microstructure. The actual microstructures comprise a phase of the principal element, such as Ti or Al, with a distributed precipitate phase. The superplastic properties of such metal alloy sheets are usually imparted by a process comprising casting, hot rolling and severe cold working to a specified sheet thickness followed by thermally-induced recrystallization to form the very fine grains in the cold worked material.
A particularly useful application of the invention is with superplastic magnesium-containing aluminum alloys, like AA5083, because of the need for relatively thick section, light weight automotive structures. Aluminum Alloy 5083 has a typical nominal composition, by weight, of about 4% to 5% magnesium, 0.3% to 1% manganese, a maximum of 0.25% chromium, about 0.1% copper, up to about 0.3% iron, up to about 0.2% silicon, and the balance substantially all aluminum. Generally, the alloy is first hot and then cold rolled to a thickness from about one to about four millimeters. In the AA5083 alloys, the microstructure is characterized by a principal phase of a solid solution of magnesium in aluminum with well-distributed, finely dispersed particles of intermetallic compounds containing the minor alloying constituents, such as Al6Mn.
In accordance with the invention, it has been repeatedly demonstrated that two 1.5 mm thick AA5083 sheets can be laid over each other congruently and stretch formed at 500xc2x0 C. with greater defect-free deformation and elongation than a single 3.0 mm thick sheet of the same composition and nominal thermomechanical processing history. Often, the two thinner sheets could be successfully formed more rapidly than the single sheet of equivalent total thickness. Typically, the two or more sheets, which may be identical or not, are formed together but are not diffusion bonded by the forming process. However, the sheets may be clamped, spot welded or the like, before superplastic forming to simplify handling.
After superplastic forming, the sheets may be permanently attached in any suitable manner, such as by welding, to form a unitary part. Alternatively, the formed sheet layers can be separated for usage as independent parts. Or two similar sheets can be separated, one inverted and the pair attached at their edges to form a symmetrical box configuration or a channel member. In other embodiments, one or more formed layers may be used as reinforcement sections over portions of a main part layer.
Other objects and advantages of the invention will become more apparent from a description of specific embodiments which follow.